This invention relates generally to zinc silicate phosphor for cathode ray tubes and, more particularly to green color-producing manganese activated zinc silicate phosphor.
Recently, there has been a requirement for the use of cathode ray tubes with high resolution for computer terminal display units, aircraft control system display units and the like, in which fine characters and graphic displays are used.
The phosphor screen of such high resolution cathode ray tubes must be composed of long persistence phosphor. When a cathode ray tube phosphor screen is composed of short persistence phosphor, flickering occurs on the screen, since the phosphor screen scanning speed is relatively slow. Generally, the phosphor composing the phosphor screen of such high resolution cathode ray tubes needs to have a long persistence time which is several tens to several hundreds of times longer than that of the short persistence phosphor of normal cathode ray tubes for TV. In this specification, the term "persistence time" means the time required for the luminescent brightness after cessation of excitation to fall to 10% of that at the time of excitation, that is to say, the 10% persistence time.
Hitherto, a manganese and arsenic activated zinc silicate phosphor (JEDEC No. P 39 phosphor) has been known as such a long persistence green color-producing phosphor. This phosphor is the most superior among the long persistence green colored phosphors known at present with respect to both luminescent brightness and persistence time, and it is used in great quantity.
However, in recent years, as the practical applications of cathode ray tubes have advanced, a longer persistence time than that of this P 39 phosphor has been required. The persistence of this P 39 phosphor is determined by the amount of arsenic activation. However, although the persistence time can be prolonged by increasing the arsenic activation, this is undesirable because of the problem of pollution (the toxicity of arsenic), and also because the brightness is reduced. Moreover, a phosphor which includes arsenic as an activator experiences the phenomenon called smear in which, initially, a weak persistence of 10% or less remains and causes a poor quality picture image in the cathode ray tube. For this reason, devices for prolonging persistence time without increasing the arsenic content have been disclosed in, for example, Japanese patent application Laid-Open No. 58-151322. In this disclosure, a phosphor with a long persistence was obtained, even though the amount of arsenic included is smal, by introducing antimony or bismuth into the phosphor with the arsenic. However, even with this device, arsenic, with its strong toxicity, was still included.
In prior art references, there is much research data regarding improvement of the persistence characteristics of phosphors. However, all of these references describe tubes with inadequate persistence and many are related to discharge lamps, which differ significantly from cathode ray tubes. Therefore, the prior art references were of no practical use in the achievement of this invention. For example, West German Pat. No. 834417 discloses that copper, silver, nickel, indium, cadmium and mercury may be introduced into a manganese activated zinc silicate phosphor. However, the reference relates to an ultraviolet excitation type phosphor for a gas-filled discharge lamp, which is very different from cathode ray tube phosphors. In fact, the typical phosphor described in this reference, in which the amount of 0.9.times.10.sup.-4 wt % of silver as a co-activator was introduced into a manganese-activated zinc silicate phosphor was tested in a cathode ray tube. However, although, in the case of ultra-violet excitation the persistence characteristics improved (the persistence time became longer) compared with a phosphor in which the elements had not been introduced, this improvement of persistence characteristics was not observed in the case of electron beam excitation.